In an automatic power transmission for an automotive vehicle, the gear ratios in the forward and reverse drive ranges of the transmission are selected by selectively actuating fluid operated frictional units including, for example, a high-and-reverse clutch, a forward drive clutch, a brake band and a low-and-reverse brake which are all operated by fluid pressure delivered from a hydraulic control system. In a known power transmission using these frictional unit, the forward drive clutch in particular is maintained coupled throughout the conditions in which an automatic forward drive range is established in the transmission. When forward drive clutch alone is permitted to be operative, the first or "low" gear ratio in the automatic forward drive range is established in the transmission mechanism. If the brake band is applied with the forward drive clutch maintained in the coupled condition, then a shift is made in the transmission mechanism from the first gear ratio to the second gear ratio in the automatic forward drive range. If, furthermore, the brake band is released and the high-and-reverse clutch in turn is made operative with the forward drive clutch held coupled, there is produced in the transmission mechanism an upshift from the second gear ratio to the third or "high" gear ratio in the automatic forward drive range. When, on the other hand, both of the high-and-reverse clutch and the low-and-reverse brake are made operative, a reverse drive gear position is obtained in the transmission mechanism. The high-and-reverse clutch is thus put into operation when either the third gear ratio in the automatic forward drive range or the reverse drive gear position is to be selected in the transmission mechanism.
The hydraulic control system for use in an automatic power transmission of this nature includes shift valves responsive to the road speeds of a vehicle and control upshifting and downshifting between the automatic forward drive range of the transmission system depending upon the vehicle speed. When the transmission gear shift valve is held in the automatic forward drive range position by the vehicle driver, the forward drive clutch is first coupled without respect to the vehicle speed. If the vehicle speed reaches a certain relatively high level, one of the shift valves incorporated in the hydraulic control system causes fluid pressure to be directed to the fluid operated servo unit for the brake band for effecting an upshift from the first gear ratio to the second gear ratio in the automatic forward drive range. If the vehicle speed is increased to a certain still higher level thereafter, another shift valve in the hydraulic control system is conditioned to direct fluid pressure to the high-and-reverse clutch for making an upshift from the second gear ratio to the third gear ratio in the automatic forward drive range. If the driving torque delivered from the engine becomes deficient for the vehicle speed being produced or the vehicle speed is reduced to a certain level under conditions in which the vehicle is being driven with the third forward drive gear ratio established in the power transmission, then one of the shift valves is conditioned to discharge fluid from the fluid chamber of the high-and-reverse clutch and the brake-apply fluid chamber of the servo unit for the brake band so that a downshift is automatically made between the second and third gear ratios in the automatic forward drive range.
If the release of the brake band is not completed at a proper timing after the high-and-reverse clutch has begun to couple during upshifting between the second and third gear ratios at relatively high vehicle speeds, the engine tends to race due to the deficiency of an overlap interval between the coupling of the high-and-reverse clutch and the release of the brake band. If, conversely, the release of the brake band is retarded excessively and produces a prolonged overlap interval, then the power train of the vehicle is brought into a condition tantamount to an interlocked condition so that violent mechanical shocks are produced therein. During downshifting between the second and third forward drive gear ratios, on the other hand, it is desirable that the brake band contributive to the shifting to the second gear ratio to initiated into motion slightly after the uncoupling of the high-and-reverse clutch is completed so as to enable the engine to restore the higher output speed. To make downshifting between the second and third forward drive gear ratios at relatively low vehicle speeds, furthermore, it is desirable that the neutral interval for which both of the brake band and the high-and-reverse clutch are to be held inoperative be reduced as the vehicle speed becomes lower since an increase in the engine speed resulting from a downshift is usually proportional to the vehicle speed being produced during the downshift. The present invention contemplates provision of a gearshift timing arrangement to achieve all these functions in an automatic power transmission.